In-cylinder emissions reduction techniques have been extensively explored to meet future regulated exhaust emissions standards for diesel engines. Exhaust gas recirculation (EGR), the most cost-effective way of reducing NOx levels from spark-ignition engines, is now being considered for use with compression-ignition (diesel) engines. Major constituents of exhaust gas that are recirculated include N2, CO2, water vapor, and partially burned hydrocarbons, which affects the combustion process through dilution, thermal, and chemical effects. The dilution effect is caused by the reduction in the concentration of oxygen in intake air; the thermal effect is caused by increasing the specific heat capacity of the charge; and the chemical effect results from the dissociation of CO2 and water vapor during combustion. In comparison, the dilution effect is the most influencing factor in altering the combustion process.
EGR can be achieved either by recirculating some of the exhaust leaving the engine back into the engine (known as external EGR) or by retaining a fraction of exhaust that never leaves the engine (known as internal EGR).
When compared to external EGR systems, the internal EGR method provides adequate mixing of the EGR fraction with the intake charge and maintains good distribution among all the cylinders, and effective NOx reduction without excessively increasing cylinder-specific particulate emissions.
In general, internal EGR can be achieved as a result of valve overlap between the intake and exhaust valve opening events. Depending on intake manifold boost level, exhaust gas back pressure, overlap duration and corresponding valve lifts, this fraction can vary substantially. However, during valve overlap, the internal EGR rate at low speed and load can be too high, in particular engines with high thermal load, where the boosted air is also used to cool the cylinders and/or exhaust valves.
An alternate to fixed valve timing is variable valve timing. It allows individual adjustment of valve timing as a function of speed and load by applying either an electromechanical valve train or an electro-hydraulic valve train. Both systems allow for separate control of intake and exhaust valve timing providing the flexibility to control internal EGR rate. In the case of cam driven valve train having a constant cam lobe profile optimized for certain operating conditions, compromises may result such as increased pumping losses caused by retarded exhaust cam phasing or reduced volumetric efficiency due to early intake valve closure.